As renewable energy becomes more prevalent, there is a pressing need for large-scale and low-cost electrical energy storage. Solar power offers a virtually inexhaustible energy source. However, large-scale storage of solar energy has not previously been commercialized, primarily due to high initial cost and difficulty of scale up. In addition, many solid-state photovoltaic (PV) cells suffer from issues related to the intermittent supply of power. Such intermittent supply of power may be due, for instance, to day/night cycles and/or cloud cover.
Therefore, to complement solid-state PV cells, photoelectrochemical (PEC) solar energy systems have been developed. PEC solar energy systems generally provide energy through the in situ production of a chemical fuel, such as hydrogen (H2) and/or oxygen (O2) obtained from the splitting of water. The PEC reactions that provide in situ production of fuel are typically catalyzed by semiconductors. Additionally, such chemical fuels may be subsequently combined in a fuel cell to generate electric power. A fuel such as hydrogen may also be burned in a modified internal combustion engine, including for transportation applications. Compared to heat, mechanical, pump-hydro or gravity-based storage systems, chemical fuels produced by in situ PEC reactions combine the advantages of high energy density and ease of storage.
Unfortunately, however, it has previously been difficult to combine an in situ PEC storage component into a PV system. Major difficulties and shortcomings of some prior attempts have included corrosion problems, the need for expensive catalysts (such as Pt), and/or poor storage options for hydrogen. As a result, even four decades following the seminal Fujishima-Honda discovery of catalytic water photolysis, a commercial solar water splitting system has yet to be realized.
As an alternative to photocatalytic hydrogen production, some previous efforts have employed expensive and complex systems for storing chemical energy in a non-hydrogen form, such as systems requiring metal hydride/NiOOH rechargeable batteries or the integration of a hydrogen bromide-embedded Si system into a regenerative system. However, during photocharging in some such systems, PEC reaction products are deposited onto a storage electrode as a solid, thereby presenting some of the same problems that are prevalent in conventional batteries. Namely, the duration of run time can be limited by the thickness of the electrode (typically approximately 1.5 hours at maximum power), and cycle life can be limited by the formation of dendrites during charging.
Therefore, there remains a need for improved solar energy systems, including improved PEC solar energy systems.